In conventional interferometric fiber optic gyroscopes (IFOG's), a laser through an optical "Y" junction provides a clockwise and a counter-clockwise propagating beam to the gyroscope ring. The interference pattern generated by the two counter-propagating beams in response to the inertial rate of rotation of the ring is directed by a directional coupler to a photodetector. While the amplitude of the fundamental frequency component of the detector output represents the rate of rotation of the gyroscope, it suffers from a number of shortcomings. The signal is proportional to the cosine of the inertial rate and so lacks sensitivity at small rates. Also, since the cosine is a multivalued function there is an ambiguous relationship between the actual and indicated rate: a given signal could represent many different rates, thus restricting the reliable dynamic range of the gyroscope to less than one cosine cycle. In addition, since the cosine is an even function there is also ambiguity as to polarity. Further, the indicated rate output is very sensitive to variations in optical power or electronic gains: any change in the optical power or gain can be misinterpreted as a change in rate.
To overcome these problems a periodic non-reciprocal phase modulation, such as a sinusoid, is applied to one of the counter-propagating beams. This resolves the polarity uncertainty in the indicated rate signal by effecting a gyroscope output which is proportional to the sine of the rate as opposed to the cosine. It also eliminates the sensitivity to optical power at zero rate.
However, there are still additional problems. The ambiguity is partially present because the sine is also a multivalued function. And there is still sensitivity to optical power at other than zero rates. To address these problems a serrodyne input is applied which introduces to the beam a non-reciprocal phase shift that exactly cancels the rate induced phase caused by the rotation of the gyroscope ring. The serrodyne input simuilates (to first order) an infinite ramp which would increase indefinitely; the serrodyne is reset periodically by a flyback voltage equivalent to 2.pi. radians. Since the flyback of 2.pi. radians effects a full cycle change in phase it minimizes the phase discontinuity at flyback. However, the optical phase modulator gain can drift so that at different times the serrodyne flyback voltage will result in different optical phase shifts that deviate from the desired 2.pi.. The net result is gyroscope scale factor drift. One approach to this problem involves optically summing a portion of the serrodyne phase shifted beam with a portion of non-phase shifted light. The result is an interference pattern which yields a sinusoidal function of phase shift introduced by the serrodyne. This sinusoid can be used to adjust the serrodyne flyback amplitude to once again produce a 2.pi. radian phase shift. See U.S. Pat. No. 4,662,751. However, this approach requires at least two additional optical couplers, an optical "Y" junction and an additional photodetector, and results in additional complex electronics.